Inspection apparatuses that use ultrasonic waves have been practically applied, for example, to non-destructive examinations in medicine and inspections in nuclear power plants. Because ultrasonic defect inspections can relatively easily detect, for example, defects in the interior of a material, it plays a significant role in the inspection of critical parts of structural materials. In an ultrasonic defect inspection, a voltage element such as that disclosed in Patent Document 1 is used, for example, in the transmission of ultrasonic waves.
This voltage element has a comparatively large radius of, for example, about 20 mm, and thus, the size of the apparatus is increased. Therefore, the measurement of narrow spaces and parts having a complicated shape is difficult. In addition, because the frequency band of the ultrasonic waves is limited by the particular frequency of the voltage element, there is a problem in that, for example, it is not very suitable for uses such as the image display of a member surface.
A laser ultrasonic wave method that is disclosed, for example, in Patent Document 2, has been proposed as a means for solving these problems.
This invention uses an optical fiber to irradiate laser light to an inspected object, an ultrasonic wave is generated on the surface of the inspected object by this laser light, and the ultrasonic wave that is propagated through the inspected object is detected by using the received laser light. Defects are detected by sensing the changes in these ultrasonic waves, and the depth can be identified by performing a frequency analysis of the received ultrasonic waves.
In particular, because a fine light fiber is used in the ultrasonic wave generation, the size of the apparatus can be reduced, and the apparatus can be used in the measurement of narrow spaces and for members having a complicated shape.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2000-28589    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2005-43139